'\" et
.TH OPEN_MEMSTREAM "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
open_memstream, open_wmemstream
\(em open a dynamic memory buffer stream
.SH SYNOPSIS
.LP
.nf
#include <stdio.h>
.P
FILE *open_memstream(char **\fIbufp\fP, size_t *\fIsizep\fP);
.P
#include <wchar.h>
.P
FILE *open_wmemstream(wchar_t **\fIbufp\fP, size_t *\fIsizep\fP);
.fi
.SH DESCRIPTION
The
\fIopen_memstream\fR()
and
\fIopen_wmemstream\fR()
functions shall create an I/O stream associated with a dynamically
allocated memory buffer. The stream shall be opened for writing and
shall be seekable.
.P
The stream associated with a call to
\fIopen_memstream\fR()
shall be byte-oriented.
.P
The stream associated with a call to
\fIopen_wmemstream\fR()
shall be wide-oriented.
.P
The stream shall maintain a current position in the allocated buffer
and a current buffer length. The position shall be initially set to
zero (the start of the buffer). Each write to the stream shall start at
the current position and move this position by the number of
successfully written bytes for
\fIopen_memstream\fR()
or the number of successfully written wide characters for
\fIopen_wmemstream\fR().
The length shall be initially set to zero. If a write moves the
position to a value larger than the current length, the current length
shall be set to this position. In this case a null character for
\fIopen_memstream\fR()
or a null wide character for
\fIopen_wmemstream\fR()
shall be appended to the current buffer. For both functions the
terminating null is not included in the calculation of the buffer
length.
.P
After a successful
\fIfflush\fR()
or
\fIfclose\fR(),
the pointer referenced by
.IR bufp
shall contain the address of the buffer, and the variable pointed to by
.IR sizep
shall contain the smaller of the current buffer length and the
number of bytes for
\fIopen_memstream\fR(),
or the number of wide characters for
\fIopen_wmemstream\fR(),
between the beginning of the buffer and the current file position indicator.
.P
After a successful
\fIfflush\fR()
the pointer referenced by
.IR bufp
and the variable referenced by
.IR sizep
remain valid only until the next write operation on the stream or a
call to
\fIfclose\fR().
.P
After a successful
\fIfclose\fR(),
the pointer referenced by
.IR bufp
can be passed to
\fIfree\fR().
.SH "RETURN VALUE"
Upon successful completion, these functions shall return a pointer to
the object controlling the stream. Otherwise, a null pointer shall be
returned, and
.IR errno
shall be set to indicate the error.
.SH ERRORS
These functions shall fail if:
.TP
.BR EMFILE
{STREAM_MAX}
streams are currently open in the calling process.
.P
These functions may fail if:
.TP
.BR EINVAL
.IR bufp
or
.IR sizep
are NULL.
.TP
.BR EMFILE
{FOPEN_MAX}
streams are currently open in the calling process.
.TP
.BR ENOMEM
Memory for the stream or the buffer could not be allocated.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
.sp
.RS 4
.nf

#include <stdio.h>
#include <stdlib.h>
.P
int
main (void)
{
    FILE *stream;
    char *buf;
    size_t len;
    off_t eob;
.P
    stream = open_memstream (&buf, &len);
    if (stream == NULL)
        /* handle error */ ;
    fprintf (stream, "hello my world");
    fflush (stream);
    printf ("buf=%s, len=%zu\en", buf, len);
    eob = ftello(stream);
    fseeko (stream, 0, SEEK_SET);
    fprintf (stream, "good-bye");
    fseeko (stream, eob, SEEK_SET);
    fclose (stream);
    printf ("buf=%s, len=%zu\en", buf, len);
    free (buf);
    return 0;
}
.fi
.P
.RE
.P
This program produces the following output:
.sp
.RS 4
.nf

buf=hello my world, len=14
buf=good-bye world, len=14
.fi
.P
.RE
.SH "APPLICATION USAGE"
The buffer created by these functions should be freed by the
application after closing the stream, by means of a call to
\fIfree\fR().
.SH RATIONALE
These functions are similar to
\fIfmemopen\fR()
except that the memory is always allocated dynamically by the function,
and the stream is opened only for output.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fIfclose\fR\^(\|)",
.IR "\fIfdopen\fR\^(\|)",
.IR "\fIfflush\fR\^(\|)",
.IR "\fIfmemopen\fR\^(\|)",
.IR "\fIfopen\fR\^(\|)",
.IR "\fIfree\fR\^(\|)",
.IR "\fIfreopen\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<stdio.h>\fP",
.IR "\fB<wchar.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
